1. Field
The invention relates to a joint prosthesis.
2. Background Information
A composite endoprosthesis with a metal shaft and a ceramic joint part is known from EP 0 024 442. The ceramic part has a conical or cylindrical recess. Support ribs are formed on a metal pin of the metal shaft, said pin being intended for introduction into the recess. Said ribs are plastically and elastically deformed on the fitting of the harder ceramic joint part. A press fit is thus obtained between the endoprosthesis parts.
From EP-A-0 712 617 a humeral head prosthesis is known wherein an articulation ball connected to a head cap via a shank is articulated on a shaft piece in a cavity with a hollow spherical base. To fix the articulation ball in the cavity, one or more grub screws are provided which can be screwed through the shaft piece against the articulation ball. In one exemplified embodiment, the articulation ball, which is of cut-open C-shape, is pressed together by the grub screw in order to clamp therein the shank which fits in a central bore in the articulation ball. In another exemplified embodiment, a grub screw is provided which can be screwed along the shank axis through the articulation ball and against the cavity base. With this screw the articulation ball is pressed against the opening of the cavity through which the shank projects from the cavity. In another example, projections cooperating with recesses in the ball surface are provided at the base of the cavity for indexing the position of the articulation ball in the cavity.
From WO 99/34756, which corresponds to FR-A-2 773 469, a shoulder prosthesis is known wherein a collar piece pivotable after the style of a ball joint in the shaft piece is articulated in a hemispherical recess in the shaft piece. The collar piece has a hemispherical articulation surface and a conical surface which is eccentric in relation to an axis through the ball center of said articulation surface, for the fitting of a joint cap thereon. The collar piece has a bore which is open from the cap side and has a hemispherical base. A screw with a spherical head which is introduced into the bore and which can be screwed into the shaft piece through an opening in the base of the bore is introduced into said bore. The spherical surfaces of the hemispherical recess in the shaft piece, the articulation surface on the shaft piece, the base of the bore and the screwhead must have the same center. Each two co-operating hollow and solid spherical surfaces must also be made very exactly and have the same radius. Minimal deviations from the ideal dimensions result in the collar piece being inadequately tightly connected to the shaft piece in order to reliably prevent unintentional pivoting of the collar piece relatively to the shaft piece during the use of the joint. The precision of the spherical surfaces required for the purpose has proved very difficult to achieve.
DE-U-299 18 589 has the object of precisely obviating this disadvantage of co-operating spherical surfaces. For this purpose it proposes an endoprosthesis for a shoulder wherein a rotary member is disposed in a shaft piece and is rotatable only about a first axis. A directional piece is articulated on the rotary piece for pivotal movement about just one second axis. A head cap can be disposed on the directional piece. The directional piece extends along a collar axis whichxe2x80x94thanks to the first and second axes crossing one anotherxe2x80x94can be brought like a ball joint into any desired position and secured therein.
DE-U-299 18 589 teaches avoiding the ball joint and simulating the ball joint movement by dividing it into two independent movements about two discrete axes. The articulation and friction surfaces between the shaft piece, rotary member and directional member, due to the fact that spherical surfaces were avoided, are surfaces of bodies of rotation such as a cylinder, torus and cone. These surfaces are much simpler to make with sufficient precision.
To secure the pivoting movement of the directional member relatively to the rotary member, it is proposed that the co-operating articulation surfaces should advantageously simply so correspond that linear contact takes place between them. For this purpose, the articulation surface on the rotary member can be constructed as a channel with two planar surfaces at an angle to one another. A bore in the directional member against which the spherical head of a screw presses, can also be made conical. It is further proposed that the articulation surfaces can also have edges and pins which are adapted to be pressed into the co-acting surface during the tightening of the screw. No example of this is given.
The present invention is directed to a joint prosthesis with a shaft piece for anchoring in the bone and a collar piece articulated thereon in the manner of a ball joint, and a head cap disposed on the collar piece, both to increase the production tolerances for the articulation surfaces involved in the ball joint and also increase the reliability of immobilizing the ball joint compared with the prior art.
According to the invention, in an exemplary joint prosthesis, at least one first one of the two co-operating articulation surfaces has at least one edge and/or point. In addition, the articulation surfaces are so constructed in respect of their shape that only one or more contact zones occur between the edge and/or point of the first articulation surface and the second articulation surface lying on a virtual spherical surface. In addition, the articulation surfaces are so constructed with respect to material that under the action of the pressure forces occurring during the pressure application at least the second articulation surface is plastically deformable in the contact zone by the edge and/or point of the first articulation surface.
As a result, when the pressure piece is pressed into contact, at least one edge or point of one articulation surface digs into the second articulation surface thus providing engagement of the two articulation surfaces by a toothing or clawing effect. The toothing allows the production tolerances to be raised. As a result, even if the surface shapes do not correspond, and even in the event of relatively considerable inaccuracies of dimensions, a ball joint is created which can be reliably immobilized. To pivot or turn the joint head relatively to the joint cavity once the head has been fixed, requires deformation of the material and therefore very considerable forces. Nevertheless, the joint head is guided in the joint cavity and pivotable in the manner of a ball joint as long as the pressure piece is not pressed tightly into contact but simply bears in contact.
The joint prosthesis has a shaft piece for anchoring in the bone. This is adapted to the bone in known manner and is optionally selected from a set of shaft pieces. A collar piece is articulated on the shaft piece and defines a collar axis. The collar piece is equipped to receive a head cap or part of the head cap. The joint prosthesis is also provided with a head cap which is designed for the joint socket co-operating with the head cap. The head cap is as far as possible shaped so as to approximate the distal natural cap and is optionally therefore selected also from a set of caps. The joint prosthesis also comprises at least one pressure piece for pressing the collar piece against the shaft piece and means for connecting the pressure piece and the shaft piece. For the articulation of the collar piece on the shaft piece there is formed either on the collar piece or on the shaft piece a joint cavity and on the other piece a joint head for disposing in the joint cavity. The joint head is pivotable or turnable in the joint cavity at least about two axes at right angles to one another. Depending on whether the joint socket is or is not also replaced, the joint prosthesis also has an artificial joint socket anchorable in the bone. If the natural socket is retained, the head cap co-operates therewith.
The plastic deformation of a contact zone in the joint cavity or a contact zone on the articulation surface of the joint head not only has the result that the necessary precision of the parts can lie within conventional production tolerances and no increased requirements apply to the production and checking of dimensional stability of the parts. The deformation of the articulation surface also results in a non-pivotable connection between the shaft piece and the collar piece. In the deformed zone, if the articulation surfaces are pressed, both or else just one participating surface can be deformed. If it is predominantly the collar piece which experiences deformation, this can be replaced, for example on a revision of the joint prosthesis, without it being necessary to replace the shaft piece.
If the contact zones between the base of the joint cavity and the joint head have one or more punctiform contacts, or even one or more continuous or interrupted linear contacts, the magnitude of the pressure exerted in these zones is inversely proportional to the size of the contact zones. In this way more intensive deformations can be obtained.
If one of the articulation surfaces is formed by one or more body edges or one or more body points and the other articulation surface is formed by a surface zone, the body points and body edges are pressed into the other articulation surface and deformed articulation surfaces form, the alignment of which is situated transversely of a tangent to a circle about the pivot center. This results in a toothing between the collar piece and the shaft piece, and therefore a secure connection which also withstands relatively considerable forces on the ball joint between the base and the collar piece. The surface zone can be smooth or rough. The possibilities of the meaning of smooth extend from untreated after machining, to polished. A rough surface can be grooved or porous, it can be etched, blasted, cast or machined or otherwise structured. Advantageously, it should be considered that the two articulation surfaces remain pivotable practically continuously relatively to one another. Rough surfaces are therefore more suitable for linear contact and smooth surfaces more for point contact.
In the exemplified embodiments, the contact zone is situated on a virtual spherical surface. An advantage of this is that the pivotability after the style of a ball joint between the collar piece and the shaft piece is obtained without additional steps. If one articulation surface, for example of the joint head, is a spherical surface, an articulation surface cooperating with this spherical surface zone advantageously forms edges situated transversely of the strongest torques acting on the collar piece before and after clamping. This prevents turning of the collar piece about the collar axis, thus ensuring that the selected position between the collar piece and the shaft piece is not changed during or after the compression of these two parts. Conversely, the spherical surface zone may have grooves and edges. These are also advantageously situated transversely of the strongest torque acting on the collar piece.
In addition to the small contact area zones between the shaft piece and the collar piece, it is also advantageous to provide such zones between the collar piece and the pressure piece. There is therefore advantageously at least one continuous or interrupted linear contact between the joint head and the pressure piece.